Peptides with opiate activity, termed endorphins, which are found in nervous tissue and pituitary gland, have been found in the AtT-20 mouse pituitary tumor cell line. Regulation of the biosynthesis and secretion of pituitary endorphins is being studied with this cell line. An assay for endorphin activity based on opiate-receptor mediated inhibition of adenylate cyclase activity of NG108-15 neuroblastoma x glioma hybrid cells has been developed. The major endorphin species of AtT-20 cells has been purified and resembles beta-endorphin. Glucocorticoids were found to regulate the endorphin content of AtT-20 cells.